1. Field of the Invention
The present invention relates to a process for the production of MnZn-base ferrites capable of making sure high saturation flux densities, minimized or limited core losses, and a well-balanced property between both saturation flux density and core loss.
2. Explanation of the Prior Art
In recent years, electronic equipment has seen rapid progresses in size reductions and high outputs. With this, there are progresses in the high integration and fast operation of various parts, and power source lines for power supply are required to have large currents, too. Parts such as transformers and choke coils are also required to be driven on large powers. In consideration of temperature rises due to the generation of heat upon driving, there are mounting demands on stable and reliable driving at high temperatures.
To meet such demands, ferrite materials that constitute transformers and choke coils are now required to have low core loss and high saturation flux density at operating temperatures.
High saturation flux densities require an increase in the amount of Fe in ferrites. However, increasing the amount of Fe leads to increased core losses; in general, only adjustments of ferrite-constituting compositions result in the inability to obtain sensible tradeoffs between both properties: high saturation flux density and low core loss.
For this reason, there are some methods tried in the art, wherein a ferrite material is composed of Fe2O3, MnO and ZnO as main ingredients with additives suitably selected from Si, Ca, Zr, Nb, Ta, V, Bi, Mo, Sn or the like added to the main ingredients, and the firing conditions for a process of firing the ferrite material are adjusted to obtain good properties: high saturation flux density and low core loss (for instance, see JP(A)6-267726 and Japanese Patent No. 3707781).
Generally in the ferrite material firing process, the partial pressure of oxygen (PO2) in a firing atmosphere at a high-temperature holding operation step (sintering temperature-holding step) and a cooling operation step (cooling process step) is determined and operated on the basis of an equilibrium relation equation in relation to temperature.
That is, for the operation of the partial pressure of oxygen and temperature, there is the following equilibrium relation equation (1) indicative of an equilibrium relation between the partial pressure of oxygen (PO2 in %) and the temperature (T in absolute temperature K), and an actual operation relation equation for the partial pressure of oxygen and the temperature is obtained by fixing the values of a and b.Log(PO2)=a−b/T  Eq. (1)
And in the prior art, once the optimum operation relation equation on the basis of the aforesaid equation (1) has been fixed, the firing operation from the high-temperature holding operation step (sintering-temperature holding step) to the cooling operation step (cooling process step) is fixed by the partial pressure of oxygen and temperature based on that operation relation equation, and kept going on.
However, there is no end to the demand on well-balanced properties between high saturation flux density and low core loss; there is an improved MnZn-base ferrite production process still in great demand.
The situation being like such, the invention has been made for the purpose of providing a MnZn-base ferrite production process that enables high saturation flux density to be achieved with low core loss or has a well-balanced property tradeoff between saturation flux density and core loss.